1. Field of the Invention
The invention relates to a device for detecting telephone line seizure, performing a measurement of voltage of the said telephone line across a first and a second optocoupler connected in series to the said telephone line and arranged head-to-tail, the first optocoupler making it possible to discharge a capacitor connected to the telephone line, the second optocoupler converting the discharge current into an image current in a measurement circuit.
2. Prior Art
The invention is more especially intended for a telephone apparatus or terminal capable of making a telephone call in an autonomous manner. The invention can for example be integrated into a cable-television or satellite-television network subscription module, such a module being installed at the subscriber's premises so as to be linked to the television receiver, to the cable or satellite network and to a telephone line. With such a module, the subscriber can in particular order a film of his choice by selecting it from a menu made available by a particular channel of the cable network. The module comprises a modem which despatches a corresponding request through the telephone line so that the cable network sends the chosen film to the subscriber's module. In the case where the telephone line is engaged by another user who is communicating with an apparatus connected in parallel to the line, the module must necessarily be capable of detecting that the line is engaged so as to defer the despatching of its request in order not to disturb the communication in progress. Similarly, when such an apparatus makes a call, it must also be capable of releasing the line if it detects that another apparatus is attempting a call. Conventionally, the detection of telephone line seizure consists in measuring the line voltage, this voltage being equal to around 50 volts if the line is free, and a lower value if it is engaged. When the line is engaged, if another apparatus attempts a call, this will also result in a drop in the line voltage.
It is known in particular to perform such a measurement with two optocouplers arranged head-to-tail and in series with the terminals of the telephone line. In this device, the first optocoupler is driven by a pulse generator so as to trigger discharges of a capacitor which is also connected to the line, in parallel with the optocouplers. Each discharge of the capacitor produces a discharge current which is converted by the second optocoupler into an image current reflecting the discharge current in a measurement circuit. The discharging of the capacitor being carried out at constant current, the discharge time is proportional to the voltage of the line divided by the value of the discharge current, and this discharge time is evaluated in the measurement circuit so as to ascertain the voltage of the line. The use of optocouplers makes it possible to isolate the line from the measurement circuit so as to protect the latter from any overvoltages which may be transmitted by the line. In such a device, the first optocoupler controls the value of the discharge current of the capacitor, this discharge current being proportional to the transfer coefficient of the first optocoupler.
The problem of such a layout is that the dispersion in the transfer coefficient of the first optocoupler generates dispersion in the measurement. One way of solving this defect could consist in selecting optocouplers having a well-defined transfer coefficient, this generating a significant extra manufacturing cost.